In recent years, a smart entry system evolved from a keyless entry system is achieving widespread use as a system for locking and unlocking doors of a vehicle. A keyless entry system locks and unlocks vehicle doors with a button provided on the key pressed, where the key needs to be taken out from a bag or pocket for example.
On the other hand, a smart entry system, provided with a short-distance radio communication capability between a vehicle and a key (referred to as a smart key hereinafter), locks and unlocks vehicle doors by means of radio communications between the vehicle and vehicle doors. Concretely, a user unlocks vehicle doors simply by touching a touch sensor on the vehicle with a smart key remaining in a bag or pocket while the vehicle doors are locked; the user locks vehicle doors simply by touching the touch sensor while the vehicle doors are unlocked. With a smart entry system, if a smart key is detected inside the vehicle, the engine can be started without the need of inserting the key into the key hole of the vehicle. Under such circumstances, whether a smart key is present inside or outside a vehicle needs to be properly detected.
This smart entry system is provided with more than one antennas on the vehicle and an antenna control unit that controls to determine which antenna is to be used. When the antenna control unit transmits a high-power signal to an intended antenna, a weak signal can be accordingly sent to other antennas that must be under no-signal conditions, a problem called crosstalk.
For example, to detect a smart key, when an antenna provided inside a vehicle transmits a high-power signal toward the smart key, an antenna provided outside the vehicle as well transmits a signal that is low-power but can be detected by the smart key. At this moment, if the smart key is present immediately near the outside antenna, determination and positioning is made that the smart key is present inside the vehicle. For example, in a case where a user is outside the vehicle with their back facing a door handle, with a smart key remaining in a rear pocket for instance, determination is made that the smart key is inside the vehicle. In such a state, a small child may accidentally start the engine.
To avoid such a situation, the technology disclosed in patent literature 1 for example has been devised. Patent literature 1 discloses a technology in which, while coding signals are not transmitted to the first antenna, a disturbing signal is transmitted to the second antenna to turn the second antenna into no-signal conditions.